Need help on a custom build

There are gunsmiths and then there are gunsmiths. Your gunsmith should have a reputation for building long range rifles.

Secondly, just as I advised the guy about his mauser, neglecting to check the action over for squareness can waste every penny of money you put into it. I have been down that road. If your current gunsmith did not check it, nor discuss it as a part of the cost, I would be looking at a new gunsmith. Technology has changed a lot over the years and the ability to measure has greatly improved.

Have the action trued and timed, if you want to stay with .243Win go with A 1:8 twist 5C Broughton barrel. Use an aftermarket precision ground recoil lug. Restock in a Mcmillian stock. Pillar bed it, glass in the lug and free float the barrel. You will be ammazed with the improvement in accuracy.